The present invention relates to a method for controlling the transmission power of response request messages carried out by a system for avoiding the collision of a first aircraft and a second aircraft, and such a system for avoiding the collision of aircraft.
The field of the present invention is that of systems for avoiding aircraft collisions and more particularly a known system called TCAS (Traffic Alert and Collision Avoidance System). Such a system can operate in accordance with a plurality of modes, in particular, a so-called C/A mode and an S mode. The present invention is adapted to the operation of the system in the S mode thereof.
Only for reasons of convenience, such a system is referred to in the present description as an anti-collision system. Any aircraft which uses such an anti-collision system is fitted, on the one hand, with a TCAS apparatus, the name of which is generally confused with that of the system and which is referred to in the present description, also for reasons of convenience, as an anti-collision apparatus and, on the other hand, with a transponder which can be used for other functions which do not relate directly to the present invention and which are not described for this reason.
The operation of such an anti-collision system in the S mode mentioned above is illustrated in FIG. 1. Any aircraft transmits at regular intervals beacon signals Sq referred to as “squitters” which particularly contain an address of the transmitting aircraft. When the anti-collision apparatus of an aircraft 1 receives such a signal Sq from another aircraft 2, it transmits a response request message Req towards the aircraft 2. Upon receipt of this message Req, the transponder of the aircraft 2 transmits a response Rep which, upon receipt by the anti-collision apparatus of the aircraft 1, is used by the apparatus to determine the distance which separates the aircraft 1 from the aircraft 2 (distance referred to as the Range), the relative speed (Range rate) between the two aircraft, the estimated time before a possible collision (Tau) and the angle between the heading of the aircraft 1 and the direction of the aircraft 2, further called the Bearing, etc. On the basis of the content of this response, the anti-collision apparatus of the aircraft 1 decides whether or not to continue this preliminary surveillance phase with a tracking phase of the aircraft 2 which is now referred to as being an “intruder”. If this is the case, response request messages Req are transmitted at regular time intervals by the aircraft 1 towards the intruder aircraft 2, to which the aircraft 2 replies with response messages Rep. The responses provided by the intruder aircraft 2 allow the anti-collision apparatus of the aircraft 1 to predict a possible collision and in particular to emit warnings, such as Traffic Advisories (TAs) and/or Resolution Advisories (RAs).
In order to transmit the respective messages thereof, in particular the messages Req and Rep, the anti-collision apparatus and the transponders of a TCAS anti-collision system use frequency bands of 1030 MHz and 1090 MHz. The transponders are used, as well as for the TCAS anti-collision system, for the secondary radar system SSR (Secondary Surveillance Radar) in order to reply to the requests therefrom in accordance with the same mechanism as for the TCAS anti-collision system. Furthermore, the DME signals (measurement of the oblique distance) are also transmitted in the same frequency bands. The various uses of these frequency bands may bring about interference between radiofrequency signals which disrupts the operation of the whole of these systems. Furthermore, as a result of the increase in the density of the traffic of aircraft and the increase in the quantity of information transmitted by these different systems, occurrences of interference between radiofrequency signals transmitted in the frequency bands mentioned above are more and more frequent and one of the problems raised by the TCAS anti-collision systems is to reduce these occurrences of interference as much as possible.
To this end, two known methods are proposed by the standard known as MOPS (Minimum Operational Performance Standards) DO-185B in relation to the systems for avoiding collisions and warning about TCAS traffic. They each involve acting on the power of transmission by the aircraft 1 of the response request messages Req towards the so-called intruder aircraft during the tracking phase mentioned above.
The first method, known as power programming in the standard DO-185B, involves transmitting these request messages Req at a transmission power Pt reduced in relation to a predetermined maximum power Pmax, even more so when the intruder aircraft 2 is near the aircraft 1 in question. This transmission power Pt is expressed in accordance with the following formula:Pt=Pmax+20 log(r/10),
where r is the distance (also referred to as the range) expressed in nautical miles, between the aircraft 1 in question and the intruder aircraft 2. The power Pmax is, for example, 250 Watt. This formula is applied only when the distance r (range) is less than 10 nautical miles.
It may be noted that this formula is arbitrary and that the transmission power Pt calculated in this manner depends only on the distance r between the two aircraft 1 and 2. Now, it has been found that, in practice, this formula leads to transmission powers Pt of the request messages Req which are still quite high and zones Z which are covered in this manner by the transmitted radiofrequency signals which are too extensive, which results in the number of aircraft which transmit in these zones Z still being too great. This also results in this so-called power programming method not leading, when the density of the air traffic is great, to an overall decrease of the occupation of the frequency bands used and to a reduction of the interference mentioned above.
The second method recommended by the standard DO-185B is known as an interference limiting method which involves reducing the transmission power of the request messages Req only in accordance with the number of intruder aircraft which surround the aircraft in question. The major disadvantage of this method is that, when the density of the air traffic becomes very great, the transmission power Pt may be considerably reduced so that intruder aircraft no longer receive the request messages Req and therefore no longer reply thereto. Therefore, they are no longer tracked by the anti-collision apparatus of the TCAS system.